The invention relates to an electrochemical device wherein a rechargeable and dischargeable storage element is enclosed.
In electronic equipment such as cellular phones, laptops, video cameras, and digital cameras, a surface mountable electrochemical device, for example, an electric double-layer capacitor or a lithium-ion rechargeable battery is used as a power supply suitable for memory backup, for example.
This type of electrochemical device generally includes an insulating casing having a recess section with an opening on its top face, a conductive lid blocking the recess section of the casing so that the recess section is airtight and watertight, a rechargeable and dischargeable storage element and an electrolytic solution enclosed in the recess section, a positive terminal and a negative terminal provided on a mounting face of the casing, a positive wire for electrically connecting the positive terminal and the positive side of the storage element, and a negative wire for electrically connecting the negative terminal and the negative side of the storage element (See Japanese Patent Application publication No. 2009-278068.).
The storage element disclosed in Japanese Patent Application publication No. 2009-278068 is formed by accumulating a first electrode sheet consisting of an active material and having a predetermined size, a second electrode sheet consisting of an active material and having a predetermined size, and a separate sheet consisting of an ion permeation sheet and having a predetermined size in that order. Because the external form of the separate sheet is slightly larger than that of each of the electrode sheets, the outer circumference portion of the separate sheet slightly extends outwardly with respect to the electrode sheets. The material of the first electrode sheet may be either different from or the same as that of the second electrode sheet depending on what kind of the electrochemical device is used.
The separate sheet may also be used to prevent the first and second electrode sheets from shorting one another. Moreover, the separate sheet may maintain an electrolytic solution between the first and second electrode sheets and allow an ion to move in the maintained electrolytic solution. The separate sheet may be made of fibrous porous sheets having a suitable thickness.
In the electrochemical device, the first and second electrode sheets as well as the separate sheet are impregnated with the electrolytic solution. The electrolytic solution flows slightly during recharge and discharge processes. However, the electrolytic solution contained in the electrode sheets may be electrolyzed or deteriorated during recharge and discharge processes, which causes the electrolytic solution contained in the portion of the separate sheet interposed between the electrode sheets to be drawn into the electrode sheets, thereby decreasing the amount of the electrolytic solution existing in the portion of the separate sheet between the electrode sheets even though it is extremely small amount.
If such decrease in the amount of the electrolytic solution occurs, it is advantageous that the portion of the separate sheet interposed between the electrode sheets draw a corresponding amount of an electrolytic solution from the portion of the separate sheet extending outwardly with respect to the electrode sheets. However, given that the thickness of the portion extending outwardly with respect to the electrode sheets is the same as that of the portion interposed between the electrode sheets and the liquid absorption of the portion extending outwardly is the same as that of the portion interposed between the electrode sheets, it is difficult to immediately draw the electrolytic solution from the portion extending outwardly to the portion interposed between the electrode sheets. In addition, the portion extending outwardly is impregnated with extremely small amount of electrolytic solution Therefore, the above decrease in the amount of the electrolytic solution contained in the portion of the separate sheet interposed between the electrode sheets during the recharge and discharge often occurs, the amount of the electrolytic solution contained in the portion remains short, which can cause deterioration in the recharge and discharge properties.
The term “liquid absorption” as used herein, refers to a water absorbing rate by the Byreck Method defined in JIS-L-1907. As the unit of the “liquid absorption”, mm/10 min will be used herein.